Flame-retardant coating

ABSTRACT

A flame-retardant coating composition, which includes a cross-linked thermosetting polymer and a metal hydroxide. A precursor composition for use in preparing the flame retardant-coating is also presented, wherein the precursor composition includes a cross-linked thermosetting polymer varnish, a metal hydroxide, an alcohol, and water. A method for coating an article with a flame-retardant coating layer and articles incorporating the flame-retardant coating are also presented.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in-part of International ApplicationNo. PCT/US06/35861 designating the United States and filed Sep. 15,2006, which in turn claims priority under 35 U.S.C. § 119(e) to U.S.Provisional Application Ser. No. 60/716,938, filed Sep. 15, 2005. Thedisclosures of both applications are incorporated herein by reference.

BACKGROUND OF THE INVENTION

In the past, fire proofing plastic involved embedding flame-retardantparticles directly into the resin matrix. The technique is fine formaterials which are not meant to bear load. However, the amount offlame-retardant particles needed to be effective in higher mass objects,for example plastic lumber, is too high. Disruption of the networkswithin a plastic significantly affects the strength and elastic modulus.Therefore, embedded particles are suitable for thinner products, forexample casings for electronics and dash boards for cars. However, forstructural applications of plastic a different approach must beconsidered to reduce the threat of fire.

Because fire is a surface phenomenon, coatings that sufficiently adjusta surface's characteristics in favor of fire resistance are important.Because the coating represents the only barrier between the plastic fueland a possible fire source, it must remain effective throughout thefire; delaying ignition of the plastic; and hindering propagation.

SUMMARY OF THE INVENTION

The present invention is directed to a flame-retardant coating layer,which includes a cross-linked thermosetting polymer and a metalhydroxide. Optionally, the coating layer also includes a blowing agent.A precursor composition for use in preparing the flame-retardant coatingis also presented, wherein the precursor composition includes across-linked thermosetting polymer varnish, a metal hydroxide, analcohol, and water. A method for coating an article with aflame-retardant coating layer and articles incorporating theflame-retardant coating are also presented.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows an article following coating application;

FIG. 2 provides an enlarged view of the coated article after drying;

FIG. 3 shows the coated article following a flame test;

FIG. 4 is a schematic representation of a flame test set up showing apropane torch in direct contact with a blowing agent coating on a steelsubstrate and an IR sensor on the reverse side of the substrate alignedon the same axis as the flame;

FIG. 5 is a flame test specimen showing the thickness of the steelsubstrate with the blowing agent coating prior to the flame test;

FIG. 6 is a flame test specimen showing the steel substrate with theblowing coating prior to the flame test;

FIG. 7 is a flame test specimen showing the steel substrate with theblowing agent coating after the flame test;

FIG. 8 depicts a flame test showing the steel substrate with the blowingagent coating subject to direct contact of a propane torch for tenminutes;

FIG. 9 is a temperature versus time plot for a flame test;

FIG. 10 provides Theological frequency sweep data showing a log-log plotof viscosity in Pascal seconds versus shear rate in per seconds atvarious lengths of curing time; and

FIG. 11 provides rheological frequency sweep data showing the initialviscosity in Pascal-seconds versus curing time of the specimen in days.

DETAILED DESCRIPTION OF THE INVENTION

The cross-linked thermosetting polymer can be applied in itscross-linked state or cross-linked after application upon exposure toambient conditions of light (visible or UV), heat, oxygen, or moisture.Cross-linked polymers useful in the present invention are capable ofbeing loaded with at least one metal hydroxide. Examples of suitablepolymers include polyurethanes, alkyd polyesters, cross-linkedpolyacrylates, and the like.

A composition of the present invention is formulated by combining atleast one metal hydroxide, alcohol, and water with a varnish solution ofthe polymer. The term “varnish” is defined according to itswell-understood meaning as a combination of a drying oil, resin (e.g.the cross-linked thermosetting polymer) and a solvent. The solvent baseof the varnish used in the present invention is preferably non-aqueous.A preferred amount of varnish in the precursor composition is from about25% to about 35% by weight of the total composition. A preferred amountof polymer in the final coating is from about 40% to about 60% by weightof the final coating.

Suitable metal hydroxides for inclusion in the polymer are those capableof providing a flame-retardant effect. For example, magnesium hydroxide(Mg(OH)₂) is a non-toxic flame retardant whose decomposition isendothermic and follows the reaction below:Mg(OH)₂→MgO+H₂OΔH=−1372.35 joules (1.30 BTU) per gramT_(c)=330°°C.The products of decomposition are non-toxic. Further, both magnesiumhydroxide and magnesium oxide are alkaline, which can reduce acidic,corrosive gas emissions from a substrate coated with the composition ofthe present invention. Water, produced by the decomposition, has thepotential to change to steam: absorbing 2259.36 joules (2.14 BTU) pergram at 100° . The steam interferes with oxygen gas, displacing it andreducing flammability. Additionally, the magnesium oxide ceramic acts asa thermal barrier, blocking and deflecting heat and flame frompenetrating to the substrate below the coating. Aluminum hydroxide(Al(OH)₃) behaves in the same manner. Therefore, preferred metalhydroxides include Mg(OH)₂ and Al(OH)₃. A preferred amount of metalhydroxide in the precursor composition is from about 25% to about 35% byweight of the total composition. A preferred amount of metal hydroxidein the final coating is from about 40% to about 60% by weight of thefinal coating.

Alcohol useful in the present invention includes, for example, isopropylalcohol, 2-propanol, and the like. A preferred amount of alcohol in theprecursor composition is from about 20% to about 30% by weight of thetotal composition.

Alternatively, regular over-the-counter rubbing alcohol (70% 2-propanolby volume) can also be used as long as one takes the water content intoaccount by quantifying the amount of water in the alcohol solution. Ifnecessary, water is added to the precursor composition in addition tothe water contributed by the alcohol solution. Similarly, magnesiumhydroxide may be used in slurry form just so long as one determines thewater content of the slurry and adds any necessary additional water tothe precursor composition. The total water content of the precursorcomposition, including water contributed by an alcohol solution and/or amagnesium oxide slurry, is from about 15% to about 25% by weight of thetotal composition.

Optionally, a blowing agent is combined with at least one metalhydroxide and a varnish solution of the polymer to form a compositionaccording to the present invention. A preferred amount of varnish in ablowing agent-containing precursor composition is from about 45% toabout 65% by weight of the total composition, with a more preferredamount being 52% by weight of the total composition. A preferred amountof polymer in the final coating is from about 45% to about 65% by weightof the final coating, with a more preferred amount being 50% by weightof the total final coating composition. A preferred amount of metalhydroxide in a blowing agent-containing precursor composition is fromabout 25% to about 45% by weight of the total composition, with a morepreferred amount being 40% by weight of the total composition. Apreferred amount of metal hydroxide in the final coating is from about25% to about 45% by weight of the total composition, with a morepreferred amount being 42% by weight of the total final coatingcomposition.

It is believed that the blowing agent retards crosslinking, and thusimparts increased flexibility and toughness to the coating, while notsacrificing adhesion. The increased flexibility allows the coating tobend with a coated article at lower than typical temperatures and resistdamage from impacts. The exposed surface cures first while theunderlying layer remains viscous due to the presence of the blowingagent. Rheology results indicate that the viscosity of the underlyinglayer increases with curing time.

Suitable blowing agents for optional inclusion in the polymer includechemical and physical blowing agents. Preferred chemical and physicalblowing agents are those that do not adversely react with the othercomponents in the composition and produce approximately 200-220 cc ofgas per gram at between 300° C. and 350° C. Examples of preferredblowing agents include azodicarbonamide and water-saturated zeolites.

A blowing agent-containing precursor composition preferably includes ablowing agent in an amount of from about 6% to about 10% by weight ofthe total composition, with a more preferred amount being 8% by weightof the total composition. A blowing agent-containing coating compositionpreferably includes a blowing agent in an amount of from about 6% toabout 10% by weight of the total final coating composition, with a morepreferred amount being 8.5% by weight of the total final coatingcomposition.

Due to the viscosity of the mixtures of the present invention, heavyduty mixing equipment may be used to achieve a homogeneous state. In oneembodiment, a hammer drill is used in conjunction with a dual bladedagitator designed to blend paint and drywall mud.

Another aspect of the current invention includes an article, wherein atleast a portion is coated with a composition, which includes across-linked thermosetting polymer and a metal hydroxide. Optionally,the coating further includes a blowing agent. Another embodimentincludes an article, wherein at least a portion is coated with acomposition, which includes a cross-linked thermosetting polymer, ametal hydroxide, an alcohol, and water.

Suitable substrates for the coated article include, for example,thermoplastics, thermoplastic composites, polyethylene, wood, stone,metal, ceramics, and the like. Preferred substrates includethermoplastics and thermoplastic composites. For example, U.S. Pat. Nos.6,191,228, 5,951,940, 5,916,932, 5,789,477, and 5,298,214 disclosestructural recycled plastic lumber composites made from post-consumerand post-industrial plastics, in which polyolefins are blended withpolystyrene or a thermoplastic coated fiber material such as fiberglass.The disclosures of all five patents are incorporated herein byreference.

The coated article can have any shape or form, for example, a roundcross-section, a rectangular cross-section, an hourglass cross-section,a sheet form, or a combination thereof. Exemplary forms for plasticcomposites are disclosed in U.S. Application No. 60/486,205 filed Jul.8, 2003, U.S. Application No. 60/683,115 filed May 19, 2005, U.S.Application No. 10/563,883 filed Jan. 9, 2006, and InternationalApplication No. PCT/US06/19311 filed May 19, 2006. The disclosures ofall of which are incorporated herein by reference. In one embodiment,the article is an L-Beam, I-Beam, a C-Beam, a T-Beam, or a combinationthereof.

Exemplary articles suitable for coating with the composition of thepresent invention include, but are not limited to, railroad ties,plastic piping, lumber, sheet piling, boat hulls, pick-up truck beds,and gasoline canisters.

Another embodiment includes a method for coating an article with aflame-retardant coating layer, which includes combining a metalhydroxide with a cross-linked thermosetting polymer varnish to form amixture; combining water and an alcohol to form a solution; combiningthe mixture with the solution to form a coating precursor composition;and applying the composition to at least a portion of the articlesurface to form a coating layer, so that the coating layer developscracks after drying. One embodiment includes a coated article preparedaccording to this method.

Another method for coating an article with a flame-retardant coatinglayer, includes combining a metal hydroxide with a blowing agent to forma well-blended mixture; combining the mixture with a cross-linkingthermosetting polymer varnish to form a blowing agent-containingprecursor composition; and applying the composition to at least aportion of the article surface to form a coating layer.

In one embodiment, the coating is applied to the article by dip coating.In this embodiment, the article is immersed in the coating compositionand removed to allow excess coating composition to drain from thearticle's surface to create a uniform coating layer. The coating is thencured. The dip coating procedure can be repeated until the desiredthickness of the coating is reached. The time interval between dippingcan be adjusted to control the rate of curing of underlying layers.

In another embodiment, the coating is applied to the article byspraying. In one embodiment, the coating is sprayed onto a horizontal orvertical surface of an article and allowed to set prior to coatinganother side of the article. Spraying equipment having high suction ispreferable. When applying the coating, an even layer is not critical butthe coating should be thick enough to obstruct vision of the underlyingsurface. Spraying creates a rough texture with random features ofvarying thickness which promotes crack formation during drying. (FIG. 1)This eliminates any chance of significant spalling; rather, smallregions of coating form, due to cracking, and remain local to theirinitial sub-melt temperature positions, extending the retardant'seffectiveness.

The coating shown in FIG. 1 is relatively even, but the crater-liketopology of the surface is evident. During drying, the polymer adheresto the substrate, which anchors it. Because much of the coating is madeup of volatiles, shrinkage is substantial. The texture of the surface incombination with the coating's bond to the substrate causes cracking,which leads to the formation of “thermoset plates” or islands containingthe flame-retardant composition.

In addition, coated plastic with vertical orientation receives an addedbenefit: flame and heat, traveling up due to buoyant force, cause theplastic above the source to melt, which then flows down due to gravityand carries the flame retardant towards the source; thereby activelysuppressing and extinguishing the fire. Surface tension prevents thethermoset plates, which resemble continents, from separating from thehot liquid plastic, allowing it to “float” on the surface. (FIG. 3).Following exposure to a flame, the majority of the coating islandsremain in their initial positions. (FIG. 3). Had the article in FIG. 3not been coated, the entire front face would have dripped from thesubstrate by the test's end. It should be noted that situationsinvolving a flame traveling up along a vertical surface represent theworst case scenario. This specific case is actively suppressed just bythe nature of the coating.

The following non-limiting examples set forth herein below illustratecertain aspects of the invention.

EXAMPLES Example 1

Coating Preparation Using Magnesium Hydroxide Slurry

Isopropyl alcohol (70% isopropyl alcohol by volume), magnesium hydroxideslurry (Flomag® H, Martin Marietta Magnesia Specialties Inc., Baltimore,Md.), and polyurethane varnish (Minwax® Fast-Drying Polyurethane,Sherwin-Williams Co., Cleveland, Ohio) are mixed in a ratio of 1:1:1 byvolume. To insure that the polyurethane does not cure prematurely, theisopropyl alcohol and magnesium hydroxide slurry are mixed and keptseparately until needed.

Example 2

Coating Preparation Using Magnesium Hydroxide Powder

Polyurethane varnish is first measured out and mixed. Magnesiumhydroxide powder is added during mixing. Next, water (7.8% by weight ofcomposition) and 70% isopropyl alcohol by volume (34.7% by weight ofcomposition) are separately combined. In the third step, the liquidprepared in step two is added to the poly-urethane varnish/Mg(OH)₂mixture of step one. The mass percents of the components can be found inTable 1. TABLE 1 Percent by Weight of Component Total CompositionIsopropyl alcohol (70% v/v) 24.3% (24.3% alcohol; 10.4% water)Polyurethane varnish 27.8% Magnesium hydroxide powder 29.7% Water(7.8% + 10.4% from alcohol solution) 18.2%

Example 3

Coated Composite Plastic Plank

A six-inch square composite plastic plank was coated with thecomposition prepared in Example 2. A propane torch was applied to thecoated plastic and to an uncoated control plastic for two minutes suchthat the one-inch inner cone of the flame just reached each sample'ssurface. Brief removal of the torch, in thirty-second intervals, wasused to determine whether each sample's surface had achievedself-sustaining combustion.

Coating a sample with the composition of Example 2 prolongs the time ittakes to achieve self-sustaining combustion (Table 2). TABLE 2 Time (s)Uncoated Coated 30 N N 60 Y N 90 Y N 120 Y YY or N: Indicates self-sustaining combustion

Example 4

Coated Plastic Railroad Ties

An oxy-acetylene torch was aimed at the bottom corner of a plasticcomposite railroad tie. Two types of plastic composite railroad tieswere used: TieTek™ Tie (TieTek, Marshall, Tex.) and (2) 32%polystyrene/68% polyethylene railroad tie (“Poly”). Four of each type ofrailroad tie were prepared for fire testing, two coated and twouncoated, and assembled into three test groups as indicated below inTable 4.

Application of the torch was attempted in the following time increments:20, 35, 60, 120, and 300 seconds. However, because of malfunctioningtorches and rain, only a portion of the total tests were conducted.After each interval, the torch was removed and the ties were allowedsome time in order to determine whether they would self-extinguish. Ifthey did, the time of this occurrence was recorded. If they did not, thetime they were extinguished with water was recorded. The results arelisted below in Table 4: TABLE 4 Flame Application Duration (sec.)Treatment 20 35 60 120 300 Test Tie Product Flame Out Time (sec.) ATieTek ™ None 0  2 200 270 245 Poly None Ext Ext Ext Ext Ext B TieTek ™None # # # # # TieTek ™ FxPl # # # # 75(285) TieTek ™ Example 2 # # # # 6(260) C Poly None 0 68 Ext Ext Ext Poly FxPl 0 Ext Ext Ext Ext PolyExample 2 0 13 Ext Ext Ext(215)# - Indicates that the test was not conductedExt - Indicates that the fire was extinguished with water(time) - Indicates the torch malfunctioned and the time the flame wasprematurely removedFxPl - Indicates a proprietary aircraft manufacturer's coatingDiscussion

Tests conducted in Examples 3 and 4 indicate that the flame retardantcoating wards off the development of self-sustaining combustion.Furthermore, the 35 second on the Poly ties shows that the ties coatedwith the composition of Example 2 self-test extinguished while the Fx P1coated tie had to be put out with water. Reinforcing this point, the 300second test on TieTek™ ties shows the ties coated with the compositionof Example 2 self-extinguishing a mere 6 seconds after the torch wasremoved while the tie coated with Fx P1 took 75 seconds to selfextinguish.

Example 5

Coating Preparation with a Blowing Agent

Metal hydroxide powder (10 parts by mass) was evenly mixed with ablowing agent powder (2 parts by mass). This mixture was slowly added toa cross-linking thermoset varnish (13 parts by mass) while continuouslymixing. The optimum percent by mass composition is approximately52/40/8% of cross-linking thermoset varnish/metal hydroxide/blowingagent. This product must be either stored in a sealed container to avoidpremature curing or applied immediately.

Example 6

Application of Blowing Agent Coating

For optimum control of coating thickness, the coating was applied at therate of approximately 100 mL/ft², corresponding to coverage ofapproximately 40 ft²/gal.

Example 7

Flame Testing of Blowing Agent Coating

For flame testing, the coating was prepared according to Example 5 andapplied to a steel coupon (76 by 152 mm) in a horizontal position andallowed to cure before testing. (FIGS. 5 and 6). The flame test employedthe flame produced by a propane torch applied normal to the coated sideof the specimen. An IR sensor (Omega OS550 Series Infrared IndustrialPyrometer) was aligned on the same axis as the flame and measuredtemperature as a function of time on the reverse side of the verticalsteel coupon. A schematic of the test set-up is shown in FIG. 4. Theinner cone length of the flame was adjusted to 3.175 cm. The tip of theinner cone, the hottest part of the flame, was positioned directly onthe sample's surface 2.54 cm above the bottom edge and at the centeracross the sample width. This configuration delivered worst casescenario results for high temperature direct point heating. Theadiabatic flame temperature of propane in air is approximately 1,927°C.+/−38° C. The flame was applied for a total duration of ten minutes.(FIGS. 7-9). Temperature versus time results were compared against acontrol specimen, an uncoated steel plate, as a point of reference.

Example 8

Rheology Testing and Results for Blowing Agent Coating

The flame retardant material was prepared according to Example 5, pouredinto open containers (90 mm diameter with a depth of less than 10 mm),and allowed to cure for various lengths of time, including 0, 1, 3, 8,15, 22, and 29 days. The exposed surface began to cure and harden first,while the underlying layers remained viscous due to the presence of theblowing agent that retards cross-linking. At each time interval, thecured surface layer was peeled off, the thickness of the layer wasmeasured, and the material underneath was subjected to rheologyexperiments to determine the viscosity of the material beneath the curedsurface layer. The Day 0 material corresponded to the initial mixture,and therefore, did not have a cured surface layer. Using a TAInstruments AR 2000 rheometer with 25 mm parallel plates at roomtemperature, the linear viscoelastic region was determined by a torquesweep from 1-10,000 microNm for the material at each length of curingtime followed by a frequency sweep from 100-0.01 Hz at the correspondingpercent strain determined from the torque sweep. A Cox-Merz mathematicaltransformation of the frequency sweep data provided viscosity versusshear rate results for the material at each length of curing time of 0,1, 3, 8, 15, 22, and 29 days. Results are presented in Table 5 and FIGS.10 and 11. Results indicate that the viscosity of the material beneaththe cured surface layer increased with specimen curing time. TABLE 5Rheoligical viscosity data as a function of curing time Curing Time(days) Initial Viscosity (Pa-second) 0 7.6 1 24.9 3 76.5 8 120.3 15106.8 22 196.3 30 265.0

The foregoing examples and description of the preferred embodimentsshould be taken as illustrating, rather than as limiting the presentinvention as defined by the claims. As will be readily appreciated,numerous variations and combinations of the features set forth above canbe utilized without departing from the present invention as set forth inthe claims. Such variations are not regarded as a departure from thespirit and script of the invention, and all such variations are intendedto be included within the scope of the following claims.

1. A flame-retardant coating layer comprising a cross-linkedthermosetting polymer and a metal hydroxide.
 2. A precursor compositionfor use in preparing the flame retardant-coating of claim 1, whereinsaid precursor composition comprises a cross-linked thermosettingpolymer varnish, a metal hydroxide, an alcohol, and water.
 3. Thecomposition of claim 1, wherein said polymer is selected from the groupconsisting of polyurethane, alkyd polyester, and cross-linkedpolyacrylate.
 4. The composition of claim 2, wherein said polymer isselected from the group consisting of polyurethane, alkyd polyester, andcross-linked polyacrylate.
 5. The composition of claim 1, wherein saidmetal hydroxide is selected from the group consisting of Mg(OH)₂ andAl(OH)₃.
 6. The composition of claim 2, wherein said metal hydroxide isselected from the group consisting of Mg(OH)₂ and Al(OH)₃.
 7. Thecomposition of claim 2, wherein said alcohol is selected from the groupconsisting of isopropyl alcohol and 2-propanol.
 8. The composition ofclaim 1 further comprising a blowing agent.
 9. The composition of claim8, wherein said blowing agent is azodicarbonamide or a water-saturatedzeolite.
 10. The composition of claim 8, wherein said blowing agent ispresent in an amount from about 6% to about 10% by weight of the coatinglayer.
 11. The composition of claim 1, wherein said polymer is presentin an amount from about 40% to about 60% by weight of the coating layer.12. The composition of claim 2, wherein said polymer varnish is presentin an amount from about 25% to about 35% by weight of the composition.13. The composition of claim 1, wherein said metal hydroxide is presentin an amount from about 40% to about 60% by weight of the coating layer.14. The composition of claim 2, wherein said metal hydroxide is presentin an amount from about 25% to about 35% by weight of the composition.15. The composition of claim 2, wherein said alcohol is present in anamount from about 20% to about 30% by weight of the composition.
 16. Thecomposition of claim 2, wherein said water is present in an amount fromabout 15% to about 25% by weight of the composition.
 17. An articlecomprising at least one surface coated with the coating layer ofclaim
 1. 18. An article comprising at least one surface coated with thecomposition of claim
 2. 19. An article comprising at least one surfacecoated with the composition of claim
 8. 20. A method for coating anarticle with a flame-retardant coating layer comprising: (a) combining ametal hydroxide with a cross-linked thermosetting polymer varnish toform a mixture; (b) combining water and an alcohol to form a solution;(c) combining the mixture of step (a) with the solution of step (b) toform a coating precursor composition; and (d) applying the compositionof step (c) to at least a portion of the article surface to form acoating layer, so that said coating layer develops cracks after drying.21. A coated article prepared according to the method of claim
 20. 22.The article of claim 17, wherein said article comprises a polymericmaterial in the form of an L-Beam, I-Beam, a C-Beam, a T-Beam, or acombination thereof.
 23. The article of claim 18, wherein said articlecomprises a polymeric material in the form of an L-Beam, I-Beam, aC-Beam, a T-Beam, or a combination thereof.
 24. The article of claim 19,wherein said article comprises a polymeric material in the form of anL-Beam, I-Beam, a C-Beam, a T-Beam, or a combination thereof.
 25. Thearticle of claim 17, wherein said polymeric material of said articlecomprises a thermoplastic composite.
 26. The article of claim 18,wherein said polymeric material of said article comprises athermoplastic composite.
 27. The article of claim 19, wherein saidpolymeric material of said article comprises a thermoplastic composite.28. The article of claim 17, wherein said article comprises a railroadtie.
 29. The article of claim 17, wherein said article comprises plasticpiping.
 30. The article of claim 19, wherein said article comprises ametal.